Since debris can build up in a valve assembly over time and thereby increase valve friction, it is desirable to provide a self-cleaning valve assembly. In a preferred embodiment, the self-cleaning valve assembly of the present invention is implemented in a transmission; however, it should be appreciated that the self-cleaning valve assembly may be used with any number of other devices as well.
The industry standard for valve bore clearances on transmission control systems is 0.0008″ to 0.0023″ diametrical clearance. This allows for 0.0005″ total tolerance on the valve diameter and 0.001″ total tolerance on the valve bore. These tolerances are well established and it is difficult to reduce the tolerance further without much more expensive part processing. It has been observed that if the filtration system does not filter out debris greater than 0.0008″, there is the potential for debris to cause valve sticking. Typically, automotive filters trap debris at sizes above 0.0024″. While higher filtration levels can provide better protection, such filters plug too quickly to be practical.
Valves are particularly sensitive to debris that is about the same size as the valve clearance. Single particles can be wedged between valve and valve body, solidly sticking the valve. Another situation can occur where particles smaller than the diametral clearance can accumulate between the valve and the valve body and gradually increase friction to the point that valve stroke times are delayed or the valve momentarily sticks. Large buildups can cause friction to overcome the return spring force causing the valve to stick in a stroked position.